1. Field of the Invention
The present invention relates to a steam cooled blade wherein steam is introduced in a moving blade operating in a high temperature operating gas to cool the moving blade, and especially to cool a trailing edge portion of the moving blade through impingement cooling, so that prevention of a high temperature and maintenance of structural strength can be ensured.
2. Description of the Prior Art
In a moving blade used in a gas turbine, a low temperature compressed air is introduced into a cooling passage provided within the moving blade to cool the moving blade from its interior. Consequently, the temperature of the moving blade is lowered to or below an allowable value, thus maintaining the structural strength of the moving blade.
In such air cooling of the moving blade, cooling air is first supplied into the moving blade passages through an inner cooling passage to convectively cool the moving blade from its interior. The cooling air is then discharged into a high temperature gas flowing outside an outer periphery of the moving blade through holes provided at a leading edge portion, a blade tip portion and a trailing edge portion of the moving blade to film cool the edge or tip portions.
FIG. 3 is a longitudinal cross sectional view of a central portion of a prior art air cooled blade, wherein a compressed air passing through the blade interior cools the moving blade. The interior of the moving blade 01 contains a cooling passage 04 running in a lengthwise direction of the blade 01 between a blade root portion 02 and a blade tip portion 03. A plurality of rows of the cooling passage 04 is provided in a blade chord direction, in other words from front to rear, of the moving blade 01 and is sectioned into a plurality of systems in the blade chord direction.
A cooling air 05 is introduced into the cooling passage 04 from an air passage provided within a rotor (not shown). The cooling air then passes to an outer periphery of the blade root portion 02, which is fitted to be rotated together with the moving blade 01, via an inlet passage 010 provided within the blade root portion 02. While passing through the moving blade 01 in the lengthwise direction between the blade root portion 02 and the blade tip portion 03, the cooling an convectively cools the moving blade 01 from its interior.
A portion of the cooling air 05 entering the inlet passage 010, after convectively cooling the moving blade 01, is discharged with a high velocity into a high temperature operating gas 09, flowing outside an outer periphery of the moving blade 01, through openings 06 provided at a leading edge portion 011 of the moving blade 01 so as to make a film cooling of a blade profile portion. Also, a portion of the cooling air 05, after convectively cooling a blade trailing edge portion 012, is discharged into the high temperature operating gas 09 through holes 07 provided at the blade trailing edge portion 012 and openings 08 provided at the blade tip portion 03.
In addition, turbulator (a turbulence promoter) 014 is disposed perpendicular to a flow of the cooling air 05, within the cooling passage 04, to make the flow of the cooling air 05 turbulent so as to enhance a cooling efficiency.
As mentioned above, in the prior art air cooled blade, the blade thickness at the blade trailing edge portion 012 of the moving blade 01 is thinner than the rest of the blade for operating efficiency of the moving blade 01. Hence, the structural strength is low and a high temperature strength could damage the moving blade. Thus, an air passage of the cooling air is cooled by the use of a convection cooling structure so that a high temperature is prevented, the structural strength is maintained and a lowering of efficiency is prevented.
Further, a recent trend toward a high temperature gas turbine indicates the use of a higher temperature operating gas for further improvement of the gas turbine thermal efficiency. For this purpose, there have been attempts of using a material which maintains its strength at a higher temperatures. There have been also been attempts to use a steam cooled blade in which steam, having a high thermal capacity and therefore being able to enhance cooling efficiency, is used to cool the moving blade in place of compressed air.
However, if the steam in a steam cooled blade, after it is used for cooling, is discharged into the high temperature gas 09, thus cooling the gas as well, the thermal efficiency of the gas turbine is greatly reduced. Thus it is necessary for all of the cooling steam to be recovered in order to enhance the total thermal efficiency of a turbine plant. Thus the cooling structure used in air cooled blade cannot be used in the high temperature gas turbines. Cooling the trailing edge portion 012 of the moving blade 01 is especially difficult, as the blade thickness is relatively thin and forming the cooling passage 04 necessary for a steam flow for cooling is problematic.